By his count, and that of his presidential campaign, the answer is more than 100,000. The calculation comes primarily from counting every position added until now at four successful investments, including Staples Inc.SPLS2.72% and Sports Authority Inc., by the private-equity firm Mr. Romney founded.

Steven N. Kaplan, a finance professor at the University of Chicago who studies private equity, used another approach: counting 100% of the jobs gained and lost at Bain companies, but only until Mr. Romney left the firm in 1999. By that measure, Mr. Kaplan concluded that Mr. Romney had created "tens of thousands" of jobs.

The debate over Bain Capital and jobs turns on how many jobs were created in Mitt Romney's time there. The estimates range from 100,000 claimed by the Romney campaign, to 1,500, under a different counting method. Mark Maremont reports. AP Photo.

The different methods of counting underscore what many experts say is the futility of trying to pin a number on something that is essentially unquantifiable. Creating jobs also wasn't the aim of Bain or other private-equity firms, which measure success by returns produced for investors.

The question of jobs created by Bain, however, has become a centerpiece of his campaign, used to buttress the contention that Mr. Romney would make a better steward of the economy than President Barack Obama.

It also has become the source of hot dispute as the Obama campaign has begun airing ads asserting that, far from creating U.S. jobs, Bain Capital under Mr. Romney was a leader in sending jobs abroad—a finding Bain and the Romney campaign reject and that Mr. Romney heatedly disputed in a campaign stop in Colorado on Tuesday.

ENLARGE

Assigning jobs numbers to private-equity firms has been a fairly rare exercise among researchers, though several academics have studied the topic in the past few years to gauge the economic impact of the industry.

As a result, there is no widely accepted accounting measure. Some academic experts said the Romney campaign's 100,000-jobs count is flawed, because it gives Bain all the credit, even though other investors also played a role in the four key companies, and includes jobs added long after Mr. Romney left Bain in 1999.

"I don't think that an investor should get credit for the jobs created beyond the ones where an investor has a position in the company," said John M. Abowd, a labor economist at Cornell University who donated to the Obama 2012 campaign.

He suggests yet another way: Give Mr. Romney and Bain credit for a share of jobs created proportional to Bain's ownership stake in its investments at the time. Because Bain held minority stakes in all four firms, and sold out years ago, that method would produce a far lower total—less than 1,500.

He said his alternative isn't one an economist would use, but, like the Romney camp's count, is a common-sense approach to the topic.

But several other experts said Mr. Abowd's method contained its own flaws, including that it fails to account for the value of guidance offered by Mr. Romney and other Bain executives. A Romney campaign official echoed that criticism, saying the method was an "unorthodox" approach that implied every passive stockholder is a job creator, giving no additional credit to firms such as Bain that actively assist companies in which they invest.

The Romney campaign declined to comment further on its own calculations.

Mr. Romney often has been more careful than his supporters in presenting his job-creation claims. He gave the 100,000-jobs number at a January Republican primary debate but later agreed that many jobs were added after he left Bain, saying: "And we're only a small part of that, by the way.…We were investors to help get them going."

The example of Staples, a clear Bain success, shows how slippery these calculations can be.

Mr. Romney includes in his tally the total Staples head count today, about 88,000 jobs. Securities filings show Bain owned 7.3% of the office-supply retailer when it went public in 1989 and had sold out by early 1992, when Staples had 5,300 employees.

Thomas Stemberg, Staples's founder and former CEO, said Mr. Romney's contribution to Staples came "from his personal advice, input and efforts, not his money." Mr. Stemberg, a major backer of the Romney campaign, said he would give Mr. Romney credit for 100% of Staples's 88,000 jobs.

By Mr. Kaplan's method, Mr. Romney should be credited with the 43,000 jobs Staples had when Mr. Romney left Bain in 1999. The weighted average count would be more like 250 jobs—a total Mr. Stemberg called "absurd."

"There's no way to settle the argument" about apportioning credit for investments made more than two decades ago, said Steven J. Davis, a University of Chicago economics professor and a fellow at the American Enterprise Institute, a right-leaning think tank. "Bain Capital probably had something to do with the early success of Staples, but there were a lot of things that happened along the way that helped Staples grow that Bain had nothing to do with."

Mr. Davis, co-author of a paper on private equity's employment role, said the reality is more complex than simple scorecards, and that the industry overall helps the economy by accelerating both job creation and destruction.

Little noticed amid the debate is that the four companies at the core of Mr. Romney's 100,000-jobs claim were relatively insignificant in the context of his 15-year Bain career. The total invested in all four was about $25 million, or about 2% of the money Bain invested during Mr. Romney's tenure. The other two, in addition to Staples and Sports Authority, are Bright Horizons Family Solutions LLC, a day-care provider, with about 19,000 employees, and Steel Dynamics Inc.,STLD-1.86% a steel producer, with about 6,500 workers.

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